Table 1 gives the sizes of fire-clay flue linings ordinarily provided for boilers, furnaces, stoves, or convection heaters burning soft coal. These sizes have proved satisfactory for average flat-grate furnaces under normal conditions. Manufacturers of heating equipment usually specify certain requirements in chimney construction and will not guarantee the performance of their heaters unless these requirements are met. Therefore their recommendations should be followed when differing materially from the dimensions given in this bulletin. Height of Chimney A chimney should extend at least 3 feet above flat roofs and 2 feet above the ridge of peak roofs. Where chimneys cannot be built high enough above the ridge to prevent trouble from eddies caused by wind being deflected from the roof, a hood may be provided with the open ends parallel to the ridge. Eddies which force air down the flues may be caused by building the chimney too near trees (fig. 5, _B_) or a higher structure (fig. 6). [1] Farmers' Bulletin 1698, Heating the Farm Home, contains information on estimating the size of the heating plant needed for houses of different sizes and for determining grate areas. [Illustration: Figure 2.--Heavy masonry chimneys of this type are still being built in rural areas. The thick walls, with unlined flues, are in good condition after 75 years of continuous use.] Table 1.--_Sizes of flue linings and heights of chimneys recommended for flat-grate furnaces burning soft coal_[A] Nominal size of flue lining --------------------------------------------------- Height of Grate Round (inside chimney top area diameter) Rectangular (outside above grate (Sq. at elevation dimensions) at elevation at elevation ft.) indicated indicated indicated - ---------------- -------------------------------------- -------------- SL 2K 4K 6K Sea Level 2,000 ft 4,000 ft 6,000 ft SL 2K 4K 6K - ---------------- -------------------------------------- -------------- In In In In In In In In Ft Ft Ft Ft 1 8 8 8 10 8½ by 8½ 8½ by 8½ 8½ by 8½ 8½ by 13 22 26 32 36 2 10 10 10 10 8½ by 13 8½ by 13 8½ by 13 8½ by 13 24 29 35 41 3 10 10 12 12 8½ by 13 8½ by 13 13 by 13 13 by 13 26 33 41 49 4 12 12 12 12 13 by 13 13 by 13 13 by 13 13 by 13 30 37 45 49 5 12 12 15 15 13 by 13 13 by 13 13 by 18 18 by 18 32 37 43 52 6 15 18 18 18 18 by 18 18 by 18 20 by 20 20 by 20 30 37 47 56 7 18 18 18 18 20 by 20 20 by 20 20 by 20 20 by 20 32 41 49 64 8 18 18 18 18 20 by 20 20 by 20 20 by 20 20 by 20 35 42 56 10 [A] If anthracite is to be burned the area of the flue cross section may be reduced about 25 percent.